In recent years, a capacitor device where an electric double-layer capacitor is used has been promised as a mechanical-power source for an electric automobile or the like. In such an electric double-layer capacitor, as its electrode, an active carbon is used which has a large specific surface area and is electrochemically inactive. This electrode is immersed in an electrolyte, and an electric charge is generated on the interface between the electrode and the electrolyte. Then, this electric charge is stored in the electric double layer so that it can be used in the capacitor.
An electric double-layer capacitor has a low withstand voltage (i.e., a rated voltage) of two to three volts. Hence, a capacitor device is generally configured by several electric double-layer capacitors. Besides, in an electric double-layer capacitor, the capacitance, internal resistance or the like of each element is known to vary widely. Therefore, if a capacitor device is formed by several electric double-layer capacitors, some of such devices can be fully charged soon while some may take a long time to charge. Hence, there is a disadvantage in that each electric double-layer capacitor cannot be charged in balance.
As the art of resolving this disadvantage, a “balance circuit system” is known. FIG. 6 is a circuit diagram, showing a circuit which embodies a “balance circuit system”. The circuit shown in FIG. 6 is made up of n (which is a positive number) electric double-layer capacitors C101 to C10n which are connected in series, and n balance resistors R101 to R10n. The balance resistors R101 to R10n each have an equivalent resistance and are connected in parallel to the electric double-layer capacitors C101 to C10n. Thereby, a bias voltage applied to each electric double-layer capacitor C101 to C10n is equal, thus helping charge each electric double-layer capacitor in a well-balanced state.
Furthermore, in another circuit (refer to Japanese Utility Model Laid-Open No. 5-23527 specification) which realizes a “balance circuit system”, a balance resistor is electrically separated from an electric double-layer capacitor at a non-charge time. When the voltage between both ends of this capacitor becomes a predetermined value or above, the capacitor is connected in parallel to the balance resistor. This prevents an electric discharge at a non-charge time, thus helping store electrical energy for a long time.
Moreover, still another circuit (refer to Japanese Patent Laid-Open No. 6-343225 specification) which embodies a “balance circuit system” includes a comparison circuit which compares the voltage between both ends of an electric double-layer capacitor and a charging reference voltage, and a by-pass circuit which receives an output from the comparison circuit and by-passes a charging current for the electric double-layer capacitor. The height of the charging reference voltage is changed suitably for various uses, so that the bias voltage of the electric double-layer capacitor can be regulated.